By forming an electrostatic latent image on a recording medium such as paper, film or cloth with the aid of an electrostatic recording head and thereafter developing the medium with liquid toner, the electrostatic recorder records the image. On a record face of the recording medium, a spacer including silica particles on the order of several .mu.m in diameter is scattered. By pressing the recording medium against the electrostatic head from the opposite side with a pad roller and bringing the record face into sliding contact with the head surface under a moderate pressure, a discharge gap due to the spacer takes place at an appropriate space between the electrostatic head and the dielectric layer in the recording medium. In this state, occurrence of a discharge between electrodes corresponding to individual images of the electrostatic recording head leads to a charging of the recording medium in accordance with an item of image information, so that an electrostatic image is formed. Onto the recording medium having passed the electrostatic head, liquid toner in which toner particles are distributed is applied with a toner roller. When the liquid toner is applied onto the recording medium, toner particles charged in the opposite polarity to the electrostatic latent image are attracted to the electrostatic latent image formed on the recording medium by the electrostatic power and combined with the surface layer of the recording medium, so that the electrostatic latent image is developed.
With a single-path color electrostatic recorder, the above-mentioned electrostatic recording heads are generally provided respectively for four primary colors, cyan, magenta, yellow and black. The above-mentioned four colors are recorded only by one-time carrying, so that a color image is obtained.
As clear from the above, the formation of an electrostatic latent image with an electrostatic recording head must be precisely carried out to accomplish an appropriate image recording in the electrostatic recorder. Accordingly, it becomes important to bring the recording medium into sliding contact with the electrostatic recording head under appropriate pressure. An insufficient pressure would lead to a dropout phenomenon wherein the image to be recorded is interrupted halfway; on the other hand, an excessive press would lead to a spurious writing phenomenon that an image comes to be recorded in a portion not to be printed by nature.
With a general electrostatic recorder, a pad roller is provided at an opposed position to the surface of an electrostatic recording head, and the record face of the recording medium is pressed against the surface of the electrostatic recording head by using the pad roller with the recording medium interposed between it and the electrostatic recording head. When the recording medium is carried during the recording, the pad roller rolls and the recording medium is always kept in sliding contact with the surface of the electrostatic recording head under a constant pressure. An apparatus for pressing the recording medium against the electrostatic recording head by using an elastic member with the recording medium interposed between it and the opposed electrostatic recording head as described above is generally called a backrest. The backrest includes an apparatus for butting a semicylindrical pad alone against the surface in addition to one using a pad roller.
FIGS. 4A and 4B are sectional views showing the outline of a backrest using a pad roller 12. Electrodes for forming an electrostatic latent image are arranged in the main scan direction (perpendicular to the paper face). The electrostatic recording head 11 has main electrodes 13a, 13b and auxiliary electrodes 14a, 14b linearly embedded in the center and on the both sides thereof, respectively. The sectional shape of the electrostatic recording head 11 is an upward convex arc. The mutual space is approx. 0.2 mm and approx. 3 mm between the main electrodes and between the auxiliary electrodes. The recording medium 10 is pressed downward by the pad roller 12 serving as a backrest and is conveyed in the arrowhead direction while keeping the record face in sliding contact with the surface of the electrostatic recording head 11. The recording medium 10 is conveyed in the subscanning direction (arrowhead direction) by the winding force of the winding roller disposed the utmost downstream while kept in sliding contact with the surface of a head by using a pad roller 12 serving as a backrest. At that time, a reverse back tension is applied to the recording medium.
Incidentally, FIGS. 4A and 4B depicts a deformation of the pad roller due to the press in exaggeration. The pad roller is formed by wrapping soft rubber on the surface of a metal core and the actual deformation is considerably smaller than shown in FIGS. 4A and 4B. The electrostatic recording head itself also is bent upward convexly and accordingly the butting width of the pad roller. The electrostatic recording head is on the order of several mm.
The broken lines shown in FIGS. 4A and 4B are curves showing a change in the pressed force of the recording medium at individual positions of the recording head. As shown in FIG. 4A, the pressure applied to the recording medium reaches a peak at the center of the head in which the deformation of the pad roller is large and decreases with a departure from the peak. Also, as shown in FIG. 4B, with the movement of the pad roller 12 from the center of the head, the peak position of the pressed pressure varies.
Accordingly, when the recording medium is pressed onto the electrostatic recording head by using the pad roller, not only the pressed pressure but also the installed position becomes important. Furthermore, since the most suitable pressure for recording depends on the material and thickness of a recording medium, the electric properties of a dielectric layer, the condition of a spacer distributed on the surface, and so on, the appropriate pressure also greatly differs with the type of a recording medium.
With a conventional electrostatic recorder, however, when a recording medium is caught with the pad roller against the electrostatic recording head, the position of the pad roller is fixed and the position and pressure against the recording material cannot be changed. Accordingly, once a recorder is fabricated so as to adjust the pressure of a pad roller to a certain type of recording medium in the fabrication step, the adjustment of the pressed pressure cannot be performed by a user. Print on another recording medium requiring a different pressure without adjustment would generate an image noise such as dropout or spurious writing. To prevent this, it was necessary to readjust the apparatus at the maker side and a flexible countermeasure corresponding to the type of recording media was difficult.
On the other hand, the pressure on the head surface of a recording medium depends on the magnitude of a tension applied in the conveyance direction of the recording medium when the push due to the backrest from behind is constant. Namely, the tension applied to the recording medium is parallel to the tangential direction of the electrostatic recording head 11 and the vertical downward force of this tension contributes to the pressure on the head surface of the recording medium. Thus, with a larger tension applied to the recording medium, the pressure on the head surface increases, whereas the pressure on the head surface decreases with a smaller tension applied to the recording medium.
To achieve an appropriate image recording in an electrostatic recorder, first of all, the formation of an electrostatic latent image must be precisely conducted. For that purpose, it becomes important that the record face of the recording medium comes into sliding contact with the head surface under a moderate pressure. In other words, an insufficient pressure of the recording medium would lead to a dropout phenomenon wherein the image to be recorded is interrupted halfway; on the other hand whereas an excessive pressure would lead to a spurious writing phenomenon wherein an image comes to be recorded in a portion not to be printed by nature.
With a color electrostatic recorder of the single path system, a recording medium comes into sliding contact with surfaces of individual electrostatic recording heads provided in series, equal in number to primary colors, and the frictional force, to which the recording medium is subjected in parallel to the conveyance direction during the sliding contact with the respective electrostatic recording heads, comes to be a resistance to the winding force of a winding roller. In addition, before the recording medium which has been developed after a liquid toner is applied thereto arrives to the next electrostatic recording head, an excess of residual liquid toner is sucked and removed with a suction device. This suction operation causes a resistance to the winding force of the winding roller.
As described above, with a color electrostatic recorder of the single path system, many factors causing a resistance to the winding force in the course of conveyance of the recording medium are involved, so that the tension applied to the recording medium is small upstream and increases with an advance to the downstream side. Thus, there was a problem that a dropout was apt to occur in the color image recorded at the upstream side and a spurious writing was apt to occur in the color image recorded at the downstream side.